When the motor is started in a motor-driven device, a starting current flow proportional to the effective value of the applied voltage passes through the motor. However, a significantly large starting current passing through the motor will cause a rise in temperature that may lead to burnout in the motor or other circuit components. Accordingly, some power tools known in the art perform soft-start control for gradually increasing the voltage applied to the motor at startup.
Since the amount of the starting current is dependent on the effective voltage applied to the motor with respect to the rotational speed of the motor, as described above, in the motor, a small amount of starting current passes when the load is light and a large starting current when the load is heavy. Hence, it is unlikely that the device will generate a large starting current for a light load, such as the load produced when driving a small screw.
However, since a conventional power tool gradually increases the voltage applied to the motor at a fixed rate, even when the load is light, the time period required to complete the starting phase of the motor is longer than necessary, worsening the power tool's ability to supply power to the motor in response to trigger operations. The performance of the power tool will feel particularly poor to the user when tightening a small screw through repeated on/off trigger operations. On the other hand, when the load is larger than expected, the conventional power tool may try to pass a considerably large amount of starting current to drive the motor, even during soft-start control, producing a rise in temperature that may cause burnout in the motor or circuit components.